Chemical modification of polymers by grafting with vinyl monomers has grained considerable interest during the last decades. Chemical modification of polymers is desired to impart particular and desirable properties, such as different types of charges, positive, negative, or both, antistatic properties, increase in moisture regain, increase in weather or alkali resistance, and in general improved mechanical properties, as well as thermal and stability properties, often can be imparted to a polymer by grafting onto the polymer different monomers.
Generally, the grafting of monomers onto polymers is achieved through several different mechanisms, including ionic mechanisms and chemical free-radical mechanisms. Each of the mechanisms offers various advantages and disadvantages. For example, the ionic technique has the advantage of having high grafting efficiency above about 90%. However, ionic techniques involve expensive solvent recovery, polymer concentration for grafting often is quite low, and the polymers employed must be very pure and completely dry. Such ionic systems are also usually uneconomical to develop on a large scale and are limited to polymers having tertiary-chloride benzylic or allylic chlorine groups. Free-radical mechanisms through irradiation provide for high grafting yield and higher grafting efficiency. However, the disadvantages of such mechanisms is that expensive solvent recovery is involved, there is possible degradation of the polymer due to the irradiation, and the Cobalt 60 or electron accelerator equipment employed in such irradiation techniques are costly and are not easily available on an industrial scale; thus, leading to an expensive overall process.
There are two general methods of chemical free-radical grafting to provide for graft polymers. One technique, called the "solution method", provides for dissolving the polymer either in a solvent or a monomer to be grafted, and then polymerization is carried out using peroxide as a free-radical source. This solution method provides for a very small amount of grafting, and, further, the grafting efficiency is very low, while the polymer often has limited solubility in the solvent or monomer; therefore, resulting in an expensive and limited process.
The other technique, known as the "dry grafting process", as disclosed in German Pat. No. 25 59 260 of July 14, 1977, suggests the employment of both monomer and initiators to be applied to a dry polymer powder on which the monomer is to be grafted. While this method would have practical advantages; that is, no solvent and no purification of the polymer, it has been found to give very low levels of grafting with accompanying low grafting yield.
Therefore, it is most desirable to provide a grafting method and graft polymers, particularly those polymers suitable for employment as membranes in ultrafiltration and reverse-osmosis processes and for employment for the separation of various substances, such as oil-in-water emulsions, aqueous paint, both anionic and cationic paint, lignosulfonate separations, concentration of polymer latices, separation of polyvinyl alcohol, desalinization of water, separation and recovery of whey and other techniques.